Magnetic resonance imaging (MRI) is a known technique in which an object is placed in a spatially varying magnetic field and subjected to pulses of radiation at a frequency which causes nuclear magnetic resonance in the object, the spectra obtained thereby being processed numerically to form cross-sectional images of the object. MRI imaging is especially useful for medical or veterinary applications, because different living tissues emit different characteristics of resonance signals, thus enabling visualization of the different living tissues in the obtained image. An MRI apparatus thus operates in general by the application of a radio frequency (RF) electromagnetic field in the presence of other magnetic fields, and the subsequent sensing and analysis of the resulting nuclear magnetic resonances induced in the body.
Conventional MRI systems include a main magnet which generates a strong static magnetic field of a high temporal stability and a high spatial homogeneity within a field-of-view (FOV) where the imaging takes place. Conventional MRI systems also include a gradient coil assembly located in the bore between the main magnet and the RF coil and generating space-varying fields. The gradient coil assembly causes the response frequency and phase of the nuclei of the patient body to depend upon position within the FOV thus providing a spatial encoding of the body-emitted signal. Conventional MRI systems further include RF coil/coils arranged within the bore which emit RF waves and receive resonance signal from the body. The superconducting main magnet is typically used to achieve high field strength; it comprises a plurality of concentric coils placed inside a cryostat which is designed to provide a low temperature operating environment for superconducting coils.
Specialized neonatal and orthopedic MRI is intended for imaging human extremities, both legs and arms, and newborn children. Its design takes advantage of small diameter of the bore compared with that in the whole body MRI that has been designed specially for adults. The magnet has to have small size and weight, low fringe field and be easily sitable in an e.g., orthopedic office. For the orthopedic MRI, very important is the short length of the magnet, i.e. the distance from its end (flange) to the imaging center, so that both knee and elbow can reach the FOV and be imaged. The shorter the distance to the FOV, the higher is the percentage of the patient and cases which could be covered without referring to the whole body MRI, including children, shorter extremities, and cases of upper parts of legs and arms. Equally important is to preserve sufficient bore diameter for the access into imaging system.
The length of the magnet is determined by the axial position of the end coils, which correlates with the diameter of the end coils, which in turn determines the bore diameter for the imaging system. The bigger the end coil diameter, the longer must be the axial distance from the end coil to the FOV. To accommodate more customers or patients with thicker extremities, such as larger leg diameter, the magnet would require a longer length, which is a longer distance from the edge to the imaging region, so the cases of shorter extremities as well as images of areas close to the entrance of the imager would be excluded. A particular challenge exists in covering the portion of overweight population with thick and short extremities. More effective design allows achieving higher feasible parameters e.g. higher field; lower cost etc. Every magnet design is a trade-off in parametric space; therefore relaxing one requirement (e.g. diameter at magnet end) allows gains in other areas.
For the reasons stated above, and for other reasons stated below which will become apparent to those skilled in the art upon reading and understanding the present specification, there is a need in the art for MRI system that can accommodate both thicker and shorter extremities with the same imaging system. There is also a need for improved access with FOV adjustments that maintain coil efficiency without the need for costly switching.